This invention relates to the art of seismic signal processing. More specifically, the invention relates to migration and velocity analysis of converted wave data.
Current methodologies for velocity analysis or imaging p-s seismic data (a.k.a. "converted wave" data) require knowledge of p-wave velocity to get a converted s-wave velocity. However, one of the reasons for using converted waves is that s-waves can be detected in some areas where there is no p-wave data. Thus, accurate knowledge of the p-wave velocity cannot be derived from the data. Therefore, an estimate of the p-wave going down and the s-wave coming up is made to calculate a Vp/Vs ratio. Events on a gather are viewed, and a velocity is picked for the "move-out" that would best fit the actual event in the data with a curve.
In a specific example, seen in FIG. 1, a p-wave section is seen in which a gas cloud 10 is present. No p-wave velocity picking can be done in that area. FIG. 2 is a p-s image, showing that the s-wave data travels through the gas cloud. The velocity picked is, effectively, a weighed average of the down-going and up-going fields, but it is not known whether the ratio of the s-wave velocity Vs to the p-wave velocity Vp is 40/60, 50/50, or something else.
In some cases, where there is p-wave data available, a Vs/Vp ratio is estimated by determining Vp from the p-p data (e.g. from a hydrophone or vertical geophone in a multicomponent system). In the alternative, Vp data from nearby well logs might be used. When p-p data is used, a time equation that is dependent only on Vp is compared to events in the data. Vp is varied, and the Vp where there is maximal energy is chosen as the correct Vp. Converted wave data is then viewed (for example, from a horizontal component of a multicomponent detector), and a different equation is used. That equation is dependent upon both Vp and Vs. Vp is presumed to be what was found from the vertical geophone or hydrophone. Then, Vs is varied, and the Vs where there is maximum energy is chosen as Vs. If Vp is wrong, Vs will be wrong. NMO, DMO, and migration are then performed. Since these processes are heavily dependent on having a correct Vp, errors can be great.
Thus, there is a need for a method and system of analyzing velocity of converted wave data in which the need for well-log data is reduced or eliminated and in which there is less dependence on accurate Vp analysis.